The SU-76 is meant to be a light tank destroyer, hence why it comes out in a tier with other light vehicles. Which is why it shouldn't scale so well, and why they have a heavier tank destroyer.
The Stug is a medium tank destroyer which is why it's available later with other medium armor. It is ostheers only tank destroyer which is why it's meant to scale vs heavier armor. It is just meant to be cheap in the same way that a T-34 is a cheap medium.
So no they aren't on the same level. One is a light vehicle the other is a medium.
I think the SU-76 isn't a light tank destroyer at all, it's a bonafide tank destroyer, just weaker and cheaper than the SU-85. The problem I see is, if it's delegated to its roll as a light TD, then it locks SU out of their actual TD until they tech up to their last tier and purchase their most expensive tank. I feel like that end-of-the-road teching just to get a TD that can fight med tanks would limit SU too much in terms of AT power. Every other faction gets their TDs relatively in the middle of the game. Jackson and Firefly only requires 2 tiers to tech up. JPIV and Stug can be gotten relatively early. Until SU spends 85 + 100 + 120 (over 300) fuel to tech up + purchase their TD, they would only have to rely on their subpar infantry AT and, in a situation where SU-76 is regulated to a light TD, their subpar SU-76 until they are at the end-of-the-end of their teching.
And SU-76 and StuG are definitely 100% comparable to each other. Labelling one a med TD and one a light TD doesn't excuse the med TD nor does it give reason for the light TD to be worse. They are both TDs that can be gotten within minutes of each other, need comparatively the same fuel to tech up to, and generally cost the same. That makes them comparable in every sense of the word. And like the person you quoted said, the StuG is miles and away better than the SU-76 for only 15 more fuel.
Also, what is even a "light TD"? Who would buy a TD that could only fight light vehicles and bother medium armor? What role would it even serve? It would be garbage again if you want it to only fight light vehicles. Honestly, the very notion makes me balk.
That's why I feel like SU-76 needs to be as capable as it is now. It already dies in 3 hits. It already does 40 less dmg than other TDs. Those are its weaknesses.